S7-1200 Torque Limiting with Festo CMMT: Telegram 750 Setup

The SIMATIC S7-1200 does not expose a native MC_TorqueLimiting technology object in the same form as S7-1500T, but torque limiting is achievable through PROFINET Telegram 750, Siemens-supplied function blocks, and intelligent drive-side handling on the Festo CMMT-AS servo drive. This reference describes the complete architecture, parameter mapping, Festo commissioning, Siemens FB wiring, and verification procedure for a linear axis application requiring ~50 N force limit.

1. System Overview

The application consists of a linear axis driven by a servo motor and controlled by a Festo CMMT-AS servo drive, supervised by a SIMATIC S7-1215 DC/DC/DC (firmware V4.6 or later recommended). The PLC sets torque limits via PROFINET using Siemens standard telegram 750, which carries the torque limit setpoint to the drive. The drive itself monitors actual torque (motor current) against the limit and either reduces command velocity or stops the axis when the threshold is exceeded.

1.1 Topology

1.2 Component List

2. Prerequisites

• S7-1200 CPU firmware V4.4 or later (V4.6+ recommended for G2 series and full S7-1200 G2 function-block support).
• Festo CMMT-AS firmware with PROFIdrive profile enabled (default on current production units).
• TIA Portal project with the Festo CMMT GSDML file imported (e.g. GSDML-V2.34-Festo-CMMT-AS-...).
• GSDML/GSD file for the CMMT available from Festo support or the Festo Support Portal.
• Siemens application manual for S7-1200 G2 torque limiting with SINAMICS (109988351) referenced as SIMATIC S7-1200 G2: Torque limiting with SINAMICS Drives.
• Reference architecture for S7-1500 force/torque limiting at Siemens TIA Portal V20 documentation: Force/Torque limiting (S7-1500/S7-1500T) for parameter semantics when applying the same concept to S7-1200.

3. Why Drive-Side, Not PLC-Side

The S7-1200 OB1 cycle time is typically 1–10 ms depending on program size. Motion control OB (MC-Servo) typically runs at 4 ms. The Festo CMMT-AS current/torque loop runs at 250 µs with field-oriented control. Any torque-limiting decision made on the PLC must travel across PROFINET (1–2 ms update) and into the drive before the drive can react. Total latency from "excess torque detected" to "drive stops" via the PLC exceeds 10 ms. By the time the PLC command reaches the drive, the motor has already been producing peak torque for several milliseconds—long enough to either trip a drive fault (Festo CMMT I²t monitoring) or damage the mechanical load.

The recommended architecture places torque evaluation in the drive. The PLC writes a torque limit value into telegram 750; the drive compares this against actual torque every cycle and either:

• Reduces velocity command (soft limit), or
• Trips a stop (hard limit) with configurable deceleration.

The PLC's role is to:

1. Provide the torque limit value (Nm or %, depending on drive scaling).
2. Enable/disable the limit via a control bit in telegram 750.
3. Monitor status bits in the process data for "limit active" and "limit reached".
4. React to the status by halting higher-level motion jobs.

4. PROFINET Telegram 750 Configuration

Telegram 750 is a Siemens free-configurable telegram used for vendor-specific I/O on PROFIdrive-compatible drives. The Festo CMMT-AS supports telegram 750 as a "manufacturer-specific" slot for transmitting torque limits and reading torque status.

4.1 Telegram 750 Signal Layout

4.2 Selecting Telegram 750 in TIA Portal

1. Open the device configuration of the Festo CMMT in TIA Portal.
2. Navigate to Properties > PROFINET interface > Module parameters.
3. In Slot configuration, add module Free PROFIdrive telegram 750 with the configured I/O length (default 8 words each direction).
4. Confirm the slot is mapped under Submodule list; the start address is automatically assigned (e.g. I/QW 256..271).
5. Compile and download the device configuration.

4.3 Configuring Telegram 750 in Festo Automation Suite

1. Open Festo Automation Suite and connect to the CMMT-AS.
2. Navigate to Parameters > PROFIdrive > Telegram selection.
3. Select Telegram 750 (vendor-specific).
4. Configure the torque limit scaling (e.g. 1000 = 100% rated torque).
5. Set reaction on limit exceed: velocity reduction (default) or rapid stop.
6. Save parameters to the drive (Save > Save to device + Save to project).

5. S7-1200 G2 Function Block for Torque Limiting

The Siemens application manual SIMATIC S7-1200 G2: Torque limiting with SINAMICS Drives (FB version V1.0.0) provides a ready-made SCL function block for torque limiting on the S7-1200 / S7-1200 G2 platform. Although the reference integration is with SINAMICS drives, the same telegram-750-based logic is applicable to any PROFIdrive-compatible drive including the Festo CMMT, provided the telegram mapping is identical.

5.1 Function Block Interface

5.2 SCL Implementation Skeleton

The following SCL code shows the typical structure for a torque-limiting FB on S7-1200. Adapt the fieldbus write/read offsets and the bit assignments to match your exact telegram-750 mapping.

FUNCTION_BLOCK "FB_TorqueLimit_S7_1200"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 1.0

VAR_INPUT
    enable         : BOOL;          // Master enable
    limitPositive  : INT;           // 0.1% units, signed
    limitNegative  : INT;           // 0.1% units, signed
    hwTorqueSlot   : HW_IO;         // Telegram 750 HW identifier
    driveReady     : BOOL;          // Drive ready feedback (STW1 bit, e.g. ZSW1.2)
END_VAR

VAR_OUTPUT
    active         : BOOL;          // Limit currently enforced
    limitReached   : BOOL;          // Drive reported "limit reached"
    actualTorque   : INT;           // Actual torque 0.1% units
    error          : BOOL;          // FB error
    status         : WORD;          // Status / diagnostic word
END_VAR

VAR
    controlWord    : WORD;          // Telegram 750 word 0 (PLC->drive)
    statusWord     : WORD;          // Telegram 750 word 4 (drive->PLC)
    limitStateWord : WORD;          // Telegram 750 word 6 (drive->PLC)
    fbError        : BOOL;
END_VAR

BEGIN
    // Default outputs
    active       := FALSE;
    limitReached := FALSE;
    actualTorque := 0;
    error        := FALSE;
    status       := 16#0000;

    IF enable AND driveReady THEN
        // Build control word: bit 0 = enable limit, bit 1 = ramp
        controlWord.%X0 := TRUE;            // enable torque limit
        controlWord.%X1 := TRUE;            // soft ramp engaged

        // Write 8-word telegram to drive via direct I/O access
        %QW0  := controlWord;               // PZD 0 - control
        %QW2  := WORD(UINT(limitPositive)); // PZD 1 - limit +
        %QW4  := WORD(UINT(limitNegative)); // PZD 2 - limit -
        %QW6  := 16#0000;                   // PZD 3 - reserved

        // Read 8-word telegram from drive
        statusWord     := %IW0;             // PZD 0 - status
        actualTorque   := INT(%IW2);        // PZD 1 - actual torque
        limitStateWord := %IW4;             // PZD 2 - limit status

        active       := statusWord.%X0;
        limitReached := limitStateWord.%X0;
        status       := statusWord;

    ELSIF NOT driveReady THEN
        // Drive not ready - clear control word, raise diagnostic
        %QW0 := 16#0000;
        %QW2 := 16#0000;
        %QW4 := 16#0000;
        %QW6 := 16#0000;
        status := 16#0001;                  // 0x0001 = drive not ready

    END_IF;

    // Catch I/O access faults (PLC runtime)
    IF ... runtime fault present ... THEN
        error := TRUE;
        status := 16#8001;
    END_IF;

END_FUNCTION_BLOCK

6. S7-1500T Reference Semantics Applied to S7-1200

The S7-1500T force/torque-limiting technology object exposes two key parameters per the S7-1500T documentation:

On S7-1200, replicate this model inside your FB:

• Mode = 0 (drive enforces, reduces velocity): Set controlWord.%X0, leave stop-bit clear.
• Mode = 1 (drive enforces, rapid stop on limit): Set controlWord.%X0 and controlWord.%X1 (stop-on-limit).
• Limit value: Convert your engineering-unit value (e.g. 50 N) to 0.1% of rated torque using the drive's torque constant (Nm/A) and the force-to-torque conversion (Nm = N × ball-screw lead / 2π × efficiency).

7. Converting 50 N to Telegram 750 Setpoint

Suppose the linear axis is a ball-screw-driven stage with the following parameters:

7.1 Torque Equivalent of 50 N

For a ball-screw:

M = F × h / (2π × η)

M = 50 × 0.005 / (2π × 0.9) = 0.0442 Nm

7.2 Percentage of Rated Torque

%M = M / M_N × 100 = 0.0442 / 2.4 × 100 = 1.84 %

7.3 Telegram 750 Integer Value

Setpoint = %M × 10 = 18 (integer, 0.1% units)

This means limitPositive := 18 and limitNegative := -18 in the FB. The drive will limit motor torque to 1.84% of rated, equivalent to ~50 N at the load.

8. Festo CMMT-AS Drive Configuration

8.1 Required Drive Parameters

8.2 Activating Torque Limiting on the CMMT

1. In Festo Automation Suite, navigate to Application > Torque Limiting.
2. Enable the function and select Source: Fieldbus telegram 750.
3. Set Reaction:
   • For gentle press-fit applications: Velocity reduction to zero.
   • For safety-sensitive applications: Category 1 stop (controlled, then STO).
4. Save parameters and cycle drive power if required (CMMT applies most parameters non-volatile on save).

9. PLC Program Architecture

9.1 OB1 Cyclic Call

// OB1 - main cyclic
// Instantiate the torque limit FB once per axis
"iDB_TorqueLimit_Axis1"(
    enable         := "Axis1_DriveReady",
    limitPositive  := REAL_TO_INT("Axis1_Limit_N" * 10.0 / "Axis1_ScalingFactor"),
    limitNegative  := -REAL_TO_INT("Axis1_Limit_N" * 10.0 / "Axis1_ScalingFactor"),
    hwTorqueSlot   := 256,                  // HW address of telegram 750
    driveReady     := "Axis1_DriveReady",
    active         => "Axis1_TorqueLimitActive",
    limitReached   => "Axis1_TorqueLimitReached",
    actualTorque   => "Axis1_ActualTorque",
    error          => "Axis1_TorqueLimitError",
    status         => "Axis1_TorqueLimitStatus"
);

// React to limit reached: halt higher-level motion
IF "Axis1_TorqueLimitReached" THEN
    "MC_Halt"(Axis := "TO_Axis1");
END_IF;

9.2 State Machine for 50 N Tool-Detect Use Case

9.3 HMI Integration

Bind the following tags to the HMI for diagnostics:

• Axis1_ActualTorque (INT, 0.1% units) — bar graph 0–100%.
• Axis1_TorqueLimitActive (BOOL) — green indicator.
• Axis1_TorqueLimitReached (BOOL) — red indicator with history bit.
• Axis1_TorqueLimitError (BOOL) — alarm field.
• Axis1_Limit_N (REAL) — HMI input field, writeable in operator mode.

10. Step-by-Step Commissioning

1. Hardware wiring: PROFINET cable from S7-1215 port to Festo CMMT-AS. Verify PROFINET link LED on CMMT is solid green.
2. Import GSDML in TIA Portal: Options > Manage device description files (GSD). Browse to Festo CMMT GSDML; install.
3. Add CMMT to PROFINET topology: drag from catalog under "PROFINET IO > Drives > Festo > CMMT-AS". Assign to the S7-1215 PROFINET interface.
4. Configure telegram 750: in the CMMT device view, slot 0, add submodule "Free PROFIdrive telegram 750". Confirm 8 PZD words each direction.
5. Assign IP and device name: from TIA Portal, "Assign PROFINET device name" to "cmmt-as-axis1".
6. Download TIA project to S7-1215.
7. Festo Automation Suite commissioning:
   • Run motor/axis setup wizard (commutation, encoder, homing).
   • Set current/torque limits in drive to safe maximums first.
   • Enable telegram 750 reception.
   • Set reaction on limit to "reduce velocity" for initial test.
8. Test open-loop torque limit: with drive enabled but axis not commanded to move, write limitPositive := 20 via watch table. Verify in Festo trace that drive registers the limit value.
9. Test axis stop on torque: command slow jog motion (10 mm/s) toward a fixed stop. Verify axis decelerates and stops within ~50 ms of contact.
10. Tune reaction: switch reaction between "velocity reduction" and "rapid stop" depending on application needs.
11. Save and back up: save Festo parameters to PC and to SD card on S7-1215.

11. Verification Checklist

12. Troubleshooting Matrix

13. Performance Comparison: S7-1200 vs S7-1500T

14. Specifications Summary

15. Safety Considerations

• Always commission with mechanical end-stops in place as a secondary hard stop.
• Configure drive I²t limits for thermal protection of motor and wiring.
• Use PROFINET IRT (isochronous real-time) for sub-millisecond deterministic updates if reaction latency is critical.
• Validate stop distance under worst-case velocity at the maximum mass.
• Implement HMI-side acknowledgement of limit-reached events to prevent auto-restart into a stalled condition.

16. Frequently Asked Questions